Extinguisher powders based on alkali metal bicarbonates



5 Claims. {01. 252 7 The present invention relates to compositions based on alkali metal bicarbonates, used in fire extinguishers of the dry powder type.

The extinguishing properties of alkali metal bicarbonates, and in particular of sodium bicarbonate, have been known for a number of years. Their use in fire extinguishing apparatus by projection under pressure calls for a free running powder which does not agglomerate during storage.

Refined sodium bicarbonate, in the state of very fine particles, is very suitable for filling extinguishing apparatus using powder. It has been known for a long time to improve its suitability for projection by coating it with a metallic salt of a long chain fatty acid.

It has likewise been proposed to improve the capacity of the extinguishing powder for projection, by coating it with hydrophobic substances such as fatty acids, waxes and certain plastic materials.

The addition to alkali metal bicarbonate powders of certain inert fillers in the finely divided state has permitted the improvement of their mobility to a degree varying with the characteristics of the filler. Among the fillers used, there may be mentioned talc, calcium and magnesium carbonates, silica, certain silicates, fly-ash, etc.

It is known, on the other hand, that while the extinguishing powders have a noteworthy efifect in abating flames, these powders do not form protective layers on liquid hydrocarbons and do not prevent re-ignition of zones where fire has been put out.

To prevent this re-ignition foaming agents are used which form a protective layer of foam over the extinguished zone. Extinguishing foams, which are very effective for avoiding re-ignition, are unfortunately only slightly suitable for the extinction of fires as such.

The best method of putting out a hydrocarbon fire consists in combining the advantages of powder and foam extinguishers, in putting out the fire by means of an extinguishing powder, then covering the zone where the fire extinguishing has been effected, with a layer of protective foam.

It has however been found that certain extinguishing powders have a detrimental effect on the stability of the foams. In particular most of the coating products used to improve the suitability for projection of the extinguishing powders, and especially the salts of long chain fatty acids have a bad effect on the compatibility of the coated powders with the extinguishing foams.

Thus the powders coated with metallic soaps cause the rapid collapse of foams which thus lose all their extinguishing properties.

Certain inert additives have likewise an adverse effect on the compatibility of the powders with foams.

The present invention provides extinguishing powder compositions compatible with the foams and presenting at the same time a very good suitability for projection.

3,26'Lfi3 Patented August 16, 1%66 The extinguishing compositions according to the invention comprise -98% of a finely divided alkali metal bicarbonate, advantageously sodium bicarbonate, potassium bicarbonate or mixtures thereof, 1-l9% of talc and l8% of very fine grain silica.

It has been found in fact that the conjoint use of talc and fine grain silica provides powder which offer simultaneously the properties of capacity for projection and compatibility with the extinguishing foams, at least equal and often superior to those offered separately by known powders considered as being either very mobile or very compatible.

Talc and silica used as additive must have a particle size as fine as possible. Talc the particles of which have an average diameter of less than one micron, of the order of 0.4 micron for example, is very suitable.

As for the silica, it is essential that it shall be made up of extremely fine grains, much more fine than the bicarbonate particles. The average size of the silica particles should be advantageously less than 1 micron, and preferably equal to a few hundredths of a micron at the most.

It has been found in fact that the use of coarser silica the average size of whose particles is over one microns; three microns, for example, provides powders which are only slightly mobile and compatible.

The properties of the powders according to the invention are particularly unexpected in view of the fact that each of the two additives, talc and very fine grain silica, employed alone, confer on extinguishing powders based on alkali metal bicarbonates, only mediocre properties.

Thus whatever the content of talc which is added to an uncoated sodium bicarbonate powder, a powder is never obtained possessing suitability for proper projection; very large contents of talc are necessary for obtaining a powder compatible with the foams.

On the other hand, while the addition of fine grain silica alone improves the mobility of the powders, even large quantities of this additive are unable to make the powders compatible with the foams.

By contrast, if talc and very fine grain silica are added simultaneously to finely divided alkali metal bicarbonate powders, powders are obtained which offer exceptional qualities for relatively small amounts of additive.

In particular, such powders offer a capacity for projection at least equal to and often superior to the best coated powders.

However, while these latter powders are practically incompatible with the extinguishing foams and destroy them in a very short time, the powders corresponding to compositions according to the inevntion are perfectly compatible with extinguishing foams. The latter can subsist at high temperature in the presence of the powders for a long time, practically as long as in the absence of the powder.

On the other hand, it has been found that the addition of talc and very fine grain silica causes no reduction in the extinguishing power of powders base on alkali metal bicarbonates.

The following composition may be used with advantage:

Percent Bicarbonate of sodium and/or potassium 88 Talc 9 Very fine grain silica 3 The bicarbonate usable in these compositions is an uncoated powder, composed of particles of a size as small as possible. A bicarbonate having an average particle size in the neighbourhood of ten microns is particularly suitable for carrying out the invention.

It has likewise been found that whilst the majority of hydrophobic coatings have the effect of reducing and even suppressing completely the compatibility of the coated powders with the extinguishing foams, the coating of bicarbonate by means of silicone oil permits the obtaining of powders which, while offering an even further improved capacity for projection, are compatible with the foams.

According to this particular embodiment of the invention, finely divided sodium bicarbonate is coated with at most 0.5% silicone oil, preferably 0.050.2%, calculated on the weight of bicarbonate to be coated.

This coating can be carried out advantageously according to the process of projection and whirling, in a mixer consisting of a horizontal drum in which an axial shaft, carrying paddles in the form of plough shears, rotates at the rate of 300 revolutions per minute.

Talc and very fine grain silica are then added to the coated powder, for the purpose of conferring thereon the properties of mobility and compatibility.

A powder of this type may have the following composition:

Percent Sodium bicarbonate 90.8 Silicone oil 0.2

Talc 6.0 Fine grain silica 3.0

It goes without saying that, since potassium bicarbonate offers extinguishing properties similar to those of sodium bicarbonate, all or part of the sodium bicarbonate may be replaced, if desired, by potassium bicarbonate.

The mixing of the different ingredients which form the extinguishing compositions according to the invention can be carried out in any industrial mixer suitable for working powders. A particularly well suited mixer consists of a horizontal drum in which an axial shaft carrying large paddles rotates at the rate of 300 revolutions per minute.

EXAMPLES Various experiments which illustrate the improvements in the capacity for projection of the powders and their compatibility with foams, by using the additives according to the invention, are given below.

The invention is in no way limited to the compositions given by way of example.

The capacity for projection of the extinguishing powders was tested in an apparatus for projecting powders, especially designed for the evaluation of the mobility and the aptitude for projection of extinguishing powders and which is a reduced model of the commercial apparatus.

This aparatus has been described in detail in Belgian Patent No. 602,890.

The results given in the table below are the weights of the residue remaining in the apparatus after projection under standard conditions.

The aptitude for projection of a powder, is considered to be less good, the greater the residue. A residue below grams is considered acceptable; if it is below 15 grams, the powder offers an exceptional mobility.

The degree of compatibility of the extinguishing powders with the foams is given by the measure of time required for the liquefaction, at elevated temperature, of a determined volume of foam recovered from a given quantity of the powder to be examined.

This test is carried out according to the method of U.S. Naval Research Laboratory, disclosed in the report No. 5329 by E. J. Jablonski and R. L. Gipe.

A powder is considered compatible when the time necessary for liquefying the foam in the presence of the powder, is greater than half the time measured for the foam alone.

Under the conditions of the experiments, this latter time was 14 minutes. The compatible powders are those of which the liquefying time measured was more than 7 minutes.

Table Residue Compat- Powder Composition Percent after proibility,

jection, min.

Sodium bicarbonate alone (particle size 10,. 50 6 Sonium bicarbonate. 97 23 5 Talc (particle size 0.4 1. 3 Sodium bicarbonate. 88 36 9 Talc 12 Sodium bicarbonate 97 23 5 Very fine grain silica (particle size 0.05 3 Sodium bicarbonate 95 18 5 Very fine grain silica 5 Sodium bicarbonate. 88 Talc 9 l8 9 Very fine grain silic 3 Sodium bicarbonate. 91 Talc 6 18 12 Very fine grain silica. 3 Sodium bicarbonate. 90. 95 Silicone oil 0. 05 14 10 Talc 6. 00 Very fine grain silica.. 3.00 Sodium bicarbonate 90. 9 Silicone o 0. 1 12 8 ale 6. 0 Very fine grain silica.. 3. 0 Sodium bicarbonate. 89 Talc 6 13 10 Very fine grain silica 5 The experiments given in the above draw the following conclusions:

1) The uncoated sodium bicarbonate, of which the average particle size is about 10 microns, offers a poor capacity for projection; its compatibility, although appreciable, is insufiicient;

(2) The addition of talc to this bicarbonate improves undeniably its aptitude for projection; with small quantities of talc one obtains powders which project relatively well, but are incompatible with the foams; with higher contents of talc a powder is obtained which is compatible with the foams, but has a rather poor aptitude for projection;

(3) The addition of very fine grain silica to the bicarbonate alone does not permit the obtaining of powders which are compatible with the foams; the powders thus obtained however possess a good mobility;

(4) The simultaneous addition of talc and very fine grain silica, of an average particle size of a few hundredths of a micron, provides powders which have an excellent compatibility and a very good and even outstanding aptitude for projection;

(5) The powders based on bicarbonate coated with silicone oil have an outstanding mobility as well as a good compatibility.

Iclaim:

1. Extinguishing powder compatible with fire extinguishing foams consisting of -98% of an alkali metal bicarbonate having an average particle size of the order of 10 microns selected from the group which consists of potassium bicarbonate, sodium bicarbonate and mixtures thereof, 119% talc having an average particle size of less than 1 micron, and 18% silica having an average particle size less than 1 micron, of the order of 0.05 micron.

2. Extinguishing powder compatible with fire extinguishing foams consisting of 80-98% of sodium bicarbonate having an average particle size of the order of 10 microns, 119% of talc having an average particle size of less than 1 micron and 18% of silica having an average particle size of less than 1 micron and of the order of 0.05 micron.

3. Extinguishing powder compatible with fire extinguishing foams consisting of 80-98% of an alkali metal bicarbonate having an average particle size of the order of 10 microns selected from the group which consists of sodium bicarbonate, potassium bicarbonate, and mixtable permit to 5 6 ture thereof, 149% of talc, and 18% of very fine grain References Cited by the Examiner silica, having an average particle size of less than 1 micron. UNITED STATES PATENTS 4. Extinguishing powder according to claim 3 in which 2 816 864 12/1957 Wamoch the very fine grain silica in a silica of which the average 2:901:427 8/1959 Steppe XR particle size is much smaller than that of the bicarbonate 5 3,085,944 4 /1963 Valentine 252 .3 8 5 XR particles, of the order of 0.05 micron.

5. Extinguishing powder according to claim 3 in which LEON ROSDOL Prlmary Examiner said talc is in particles having an average diameter of less JULIUS GREENWALD Exammerthan 1 micron. M. WEINBLATT, Assistant Examiner. 

3. EXTINGUISHING POWDER COMPATIBLE WITH FIRE EXTINGUISHING FOAMS CONSISTING OF 80-98% OF AN ALKALI METAL BICARBONATE HAVING AN AVERAGE PARTICLE SIZE OF THE ORDER OF 10 MICRONS SELECTED FROM THE GROUP WHICH CONSISTS OF SODIUM BICARBONATE, POTASSIUM BICARBONATE, AND MIXTURE THEROF, 1-19% OF TALC, AND 1-8% OF VERY FINE GRAIN SILICA, HAVING AN AVERAGE PARTICLE SIZE OF LESS THAN 1 MICRON. 